I have heard that the only planets able to be seen outside the Solar system are Jovian-sized planets with the occasional detection of planets three times the Earth's size. But, as far as I know, we haven't seen any Earth-sized (due to the range of our telescopes?) extrasolar planets. When will we be able to detect them?

4 Answers
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In 2013, the smallest detected exoplanet was Kepler-37-b, which is actually smaller in mass and size than Earth, so we already had a limited ability to detect these size exoplanets.

It is worth noting that there wasn't any new technology that allowed this advance, the paper linked to by NASA indicates that the same methods were used as they would usually use. Making it remarkable that they were even able to discover something this small.

Now, even more Earth-sized planets have been detected. One method used to aid in their detection is described as follows:

We account for Kepler's imperfect detectability of such planets by injecting synthetic planet-caused dimmings into the Kepler brightness measurements and recording the fraction detected.

Maybe update this answer. There are now a number of confirmed Earth-sized planets and most of the small planetary candidates found by Kepler will be real planets. So the answer is indeed that we already can and have. What there isn't yet - is an Earth-like planet in an Earth-like orbit around a sun-like star.
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Rob JeffriesMay 3 at 14:16

Planetary Transits and Oscillations of stars (PLATO) is a planned European Space Agency space observatory that will use a group of photometers to discover and characterize rocky extrasolar planets of all sizes around red dwarf stars, yellow dwarf stars like our Sun, and subgiant stars where water can exist in liquid state. . . . The goal is to find planets like Earth, not just in terms of their size but in their potential for habitability.

We can find Earth like Extra-Solar Planets, but they just might be too close to their star, and therefore, life might not be in the planet. We are right now living in a "Goldilocks Planet". Goldilocks planets are in the habitable zone, where there is life, and H2O that is at the right temperature to be a flowing liquid.

Anyway, a planet such as Gliese 876 d, is WAY too close to the star it orbits, (Gliese 876) Which it is highly unlikely that life exists, considering it is 377'C (710.6'F)

Analysis of Kepler data has yielded the smallest known mass for an exoplanet orbiting a normal star. Its mass and size are similar to those of Mars, setting a benchmark for the properties of exoplanets smaller than Earth. See Letter p.321